Influence of sediment on pelagic carbon and nitrogen turnover in a shallow Danish estuary

The impact of sediment on pelagic microbial processes was examined for 3 different sediment types (a bare sandy sediment, an eelgrass sediment and a diatom sediment) in a shallow Danish estuary. Diel and seasonal variations in the in situ fluxes and pelagic turnover of various C and N compounds were measured in plexiglas chambers incubated on top of the sediment and related to parallel control incubations without sediment contact. In April, the microbial activity was low, and the sediment had relatively little influence on the pelagic C and N turnover. In August, however, the eelgrass and diatom sediments had a significant stimulatory effect on the bacterial production, which was 2 to 4 times higher than at the bare sediment station. The measured effects of the sediment on pelagic C and N turnover, thus, appeared to be related to the presence and activity of benthic microalgae and macrophytes. Stimulation of bacterial production by the sediment was in all experiments associated with a net flux of dissolved organic carbon (DOG) from the sediment. We suggest that the release of biodegradable DOC from benthic microalgae and macrophytes caused a stimulation of the bacterioplankton production. Since 2 to 4 times more nitrogen was retained in the pelagic bacterial biomass at the eelgrass and diatom sediment than at the bare sediment location, we suggest that the presence of benthic microalgae and macrophytes may cause a temporary immobilization of N in bacterioplankton biomass.